1. Field of the Invention
The invention relates to new powder coating compositions based on polyacrylates having hydroxyl groups, polyesters having hydroxyl groups and aliphatic and/or cycloaliphatic polyisocyanates blocked with ketoximes, a process for their preparation, and their use for the preparation of coatings on heat-resistant substrates.
2. Description of the Prior Art
Oxime-blocked polyisocyanates and their use in powder coating compositions are known and have been disclosed, e.g., in European Patent Application 432,257 (WO 91/00267), U.S. Pat. No. 3,857,818, German Offenlegungsschrift 2,200,342 and Japanese Patent Application 70-94941, Publication Number 75-27057 (C.A. 84:107163s). Frequently these polyisocyanates are used in powder coating compositions in combination with polyesters having hydroxyl groups. However, the resulting coatings do not possess adequate stability towards light and weather and, thus, are not suitable, for example, for external applications for motor vehicles.
The combination of oxime-blocked polyisocyanates and copolymers having hydroxyl groups is also mentioned in some of these publications. The Japanese publication describes the combination of (i) xylylene diisocyanate having isocyanate groups which are partly blocked with a special oxime, and (ii) a polyacrylate having hydroxyl groups. Example 4 of U.S. Pat. No. 3,857,818 describes the combination of (i) a high-melting butanone oxime-blocked isocyanate prepolymer prepared from 4,4'-diisocyanato-dicyclohexylmethane and (ii) a polyacrylate having hydroxyl groups which are solid at room temperature.
These powder coating compositions suffer from the disadvantage that they may be stoved only at relatively high temperatures because their flow properties are poor at moderate temperatures due to the high melting points of the individual components. Frequently, these poor flow properties result in the formation of bubbles in the coatings due to the escape of the blocking agent. Furthermore, it is a disadvantage that only brittle, inflexible coatings are obtained by using this type of powder coating composition. These coatings are not suitable, for example, for painting automobiles where paints with tough, resilient and flexible properties are required.
Possible solutions to the inadequate flow properties and unacceptable flexibility of PUR powder coating compositions based on polyacrylates are described in U.S. Pat. Nos. 3,993,849 and 3,998,768. These patents describe polyol mixtures, consisting of polyacrylates having hydroxyl groups and polyesters with having hydroxyl groups, which are combined with oxime-blocked polyisocyanates as binder components. In addition, in U.S. Pat. No. 3,998,768 the polyacrylate component is based on one polyacrylate having a high glass transition temperature and one polyacrylate having a low glass transition temperature. In both patents, the total binder composition contains a proportion of polyester of at least 10% by weight and at most 30% by weight. These proportions of polyester, however, may have a negative effect on resistance to weathering, especially on the tendency of automotive top coats to yellow when used outside, so that powder coating composition binders in accordance with these U.S. patents are suitable for external use only to a limited extent.
Accordingly, it is an object of the present invention to provide new powder coating compositions which may be hardened at comparatively low temperatures of 120.degree. to 160.degree. C. and have excellent flowability, even at these temperatures, such that the resulting coatings are high-quality, bubble-free, and particularly resistant to solvents, chemicals, and weathering.
These objects may be achieved with the powder coating compositions according to the invention which is described in more detail hereinafter.
Surprisingly, it has been found that small proportions of polyesters having hydroxyl groups, i.e., less than 10% by weight, greatly improve the flow properties and the degree of flexibility of PUR powder coating compositions based on polyacrylates having hydroxyl groups and oxime-blocked polyisocyanates. These improvements are not obtained when using higher proportions of polyester. Instead these coatings have poorer weathering properties and an increased tendency to yellow when exposed to high temperatures.